GaN and other III-nitride alloys are a promising class of materials with favorable properties that have a broad range of technological applications. For example, the tunable direct bandgap between 0.7 and 6.1 eV make this class of materials attractive for photovoltaic and optoelectronic applications. See S. Nakamura, T. Mukai and M. Senoh, “High-Power GaN P—N Junction Blue-Light-Emitting Diodes,” Jpn. J. Appl. Phys., 30, L1998 (1991); see also J. W. J. B. Luo, F. Ren, K. K. Allums, C. R. Abernathy, S. J. Pearton, R. Dwivedi, T. N. Fogarty, R. Wilkins, A. M. Dabiran, A. M Wowchack, C. J. Polley, P. P. Chow, and A. G. Baca, “dc and rf performance of proton-irradiated AlGaN GaN high electron mobility transistors,” Appl. Phys. Lett. 79, 2196 (2001). The wide bandgap and mobilities achievable make III-nitrides suitable for power electronic applications while the radiation hardness of III-nitrides makes them suitable for extreme environments required in military and space environments.
Despite their having many promising characteristics, one of the major challenges for processing III-nitride materials, SiC, and diamond is the ability to anneal such materials at high temperatures. Thermal annealing of defects in semiconductors occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Annealing at high temperatures is a necessary step in semiconductor processing to repair implantation damage, activate implanted dopants, and repair damage induced by etching. See U.S. Pat. No. 8,518,808 to Feigelson et al., entitled “Defects Annealing and Impurities Activation in III-Nitride Compound,” which shares several inventors in common with the present invention and which is hereby expressly incorporated into the present disclosure in its entirety. The ability to repair implantation damage and activate implanted dopants is imperative for precise control of the dopant profiles. Applications where this ability will be a key enabling step include implanted guard rings, which can be used for electric field spreading in vertical GaN diodes, and implantation/activation of dopants, which can be utilized in contact regions to lower contact resistance.
However, it is difficult to anneal defects in III-nitride semiconductors such as GaN and its alloys with InN and AlN and to activate impurities in the semiconductor material after growth of the semiconductor material, and especially after the implantation of dopant ions in the material. Temperatures required for the removal of defects induced by implantation of dopants such as magnesium (Mg) and the activation of impurities after their implantation are in the range of 1400° C., but GaN is not stable at atmospheric pressure and temperatures above 850° C., instead decomposing into Ga and N2.
There are some known approaches which partially solve the problem of GaN annealing. For example, GaN is stable when annealed at temperatures above 1400° C. under gas pressures above 1.0 GPa. See S. Porowski, I. Grzegory, D. Kolesnikov, W. Lojkowski, V. Jager, W. Jager, V. Bogdanov, T. Suski and S. Krukowski, “Annealing of GaN under high pressure of nitrogen,” J. Phys., Condens. Matter., 14, 11097 (2002). However, such high gas pressures require special equipment and make such an annealing procedure not efficient for the industrial use.
In other cases, GaN can be annealed at temperatures higher than 850° C. if it is capped with a material that is more stable at high temperatures, for example, AlN. See S. Matsunaga, S. Yoshida, T. Kawaji and T. Inada, “Silicon implantation in epitaxial GaN layers: Encapsulant annealing and electrical properties,” J. Appl. Phys., 95, 2461 (2004). Such an AlN cap can be made by AlN spattering or AlN MOCVD growth on the top of a GaN sample and allows annealing of GaN at temperatures above 1000° C. without noticeable nitrogen loss from GaN. The cap suppresses an escape of nitrogen from the GaN before pressure of nitrogen in the interface the between GaN and the AlN builds up and makes small cracks in the AlN film or breaks it.
Another approach allowing to enhance GaN annealing is rapid thermal annealing (RTA). See G. S. Aluri, M. Gowda, N. A. Mahadik, S. G. Sundaresan, M. V. Rao, J. A. Schreifels, J. J. A. Freitas, S. B. Qadri and Y.-L. Tian, “Microwave annealing of Mg-implanted and in situ Be-doped GaN,” J. Appl. Phys., 108, 083103 (2010). Fast heating (in seconds) to the temperatures above 850° C. and cooling (in seconds) kinetically prevents GaN from the decomposition at temperatures above their thermodynamic stability. If high temperature is applied to GaN for a very short time, defects can be partially annealed without the GaN decomposing. The higher the temperature applied, the more different defects that can be annealed, but the shorter the annealing time that should be applied to prevent GaN from decomposing. During the fast heating only nitrogen from surface has time to leave GaN, and if RTA is combined with use of a cap, it is possible to heat GaN very fast up to 1400° C. without noticeable decomposition.
The use of RTA in combination with an AlN cap has been observed to restore the GaN structure damaged by implantation and activate implanted n-type impurities. See J. A. Fellows, Y. K. Yeo, R. L. Hengehold, and D. K. Johnstone, “Electrical activation studies of GaN implanted with Si from low to high dose,” Appl. Phys. Lett. 80, 1930 (2002).
However, this type of annealing hasn't created p-type conductivity in GaN samples implanted with Mg. The GaN lattice is highly damaged by implantation of the Mg ions and so is less stable than n-type doped or unimplanted GaN, and the time during which Mg-doped GaN can be exposed to the high temperature during RTA without decomposing is insufficient to obtain activation of the Mg ions. In addition, during non-equilibrium RTA of GaN, Mg may occupy too many available N-lattice sites and so doesn't become a p-type impurity.
To overcome the disadvantages of the known annealing approaches for GaN and other III-nitride semiconductors a new process, known as “multicycle rapid thermal annealing” (MRTA) was developed by some of the inventors of the present invention. See U.S. Pat. No. 8,518,808, supra, which describes the MRTA annealing process in detail. The MRTA process combines using a cap, applying moderate N2 overpressure, and rapidly applying multiple fast heating and cooling pulses to expose the sample to high temperatures for a sufficiently long time to obtain the diffusion processes required to remove defects and activate impurities in the material.
The efficiency of MRTA for diffusion-controlled defects annealing at temperatures above thermodynamic stability of material was demonstrated by electrical activation of Mg implanted in GaN showing for the first time p-type conductivity in Mg implanted GaN. See U.S. Pat. No. 8,518,808, supra. However, electrical conductivity along the surface of GaN after MRTA is not uniform. One of the reasons of such non-uniformity is slight degradation of the GaN crystalline quality caused by the MRTA process itself. These detrimental structural changes can be attributed to the formation of defects formed and quenched during rapid heating and cooling cycles. A new technique is required to improve MRTA.